Li-ion batteries have traditionally dominated the market of portable electronic devices. However, conventional Li-ion batteries contain flammable organic solvents as components of the electrolyte and this flammability is the basis of a safety risk which is of concern and could limit or prevent the use of Li-ion batteries for application in large scale energy storage.
Replacing the flammable organic liquid electrolyte with a solid Li-conductive phase would alleviate this safety issue, and may provide additional advantages such as improved mechanical and thermal stability. A primary function of the solid Li-conductive phase, usually called solid Li-ion conductor or solid state electrolyte, is to conduct Li+ ions from the anode side to the cathode side during discharge and from the cathode side to the anode side during charge while blocking the direct transport of electrons between electrodes within the battery.
Moreover, lithium batteries constructed with nonaqueous electrolytes are known to form dendritic lithium metal structures projecting from the anode to the cathode over repeated discharge and charge cycles. If and when such a dendrite structure projects to the cathode and shorts the battery energy is rapidly released and may initiate ignition of the organic solvent.
Therefore, there is much interest and effort focused on the discovery of new solid Li-ion conducting materials which would lead to an all solid state lithium battery. Studies in the past decades have focused mainly on ionically conducting oxides such as for example, LISICON (Li14ZnGe4O16), NASICON(Li1.3Al0.3Ti1.7(PO4)3), perovskite (for example, La0.5Li0.5TiO3), garnet (Li7La3Zr2O12), LiPON (for example, Li2.88PO3.73N0.14) and sulfides, such as, for example, Li3PS4, Li7P3S11 and LGPS (Li10GeP2S12).
While recent developments have marked the conductivity of solid Li-ion conductor to the level of 1-10 mS/cm, which is comparable to that in liquid phase electrolyte, finding new Li-ion solid state conductors is of great interest.
An effective lithium ion solid-state conductor will have a high Li+ conductivity at room temperature. Generally, the Li+ conductivity should be no less than 10−6 S/cm. Further, the activation energy of Li migration in the conductor must be low for use over a range of operation temperatures that might be encountered in the environment. Additionally, the material should have good stability against chemical, electrochemical and thermal degradation. Unlike many conventionally employed non-aqueous solvents, the solid-state conductor material should be stable to electrochemical degradation reactivity with the anode and cathode chemical composition. The material should have low grain boundary resistance for usage in an all solid-state battery. Ideally, the synthesis of the material should be easy and the cost should not be high. Unfortunately, none of the currently known lithium ion solid electrolytes meet all these criteria. For example, Li10GeP2S12 fails to meet the requirement of electrochemical stability and has a high cost due to the presence of Ge, despite its state-of-art Li conductivity. Environmentally stable composite materials having high Li+ conductivity and low activation energy would be sought in order to facilitate manufacturing methods and structure of the battery.
The standard redox potential of Li/Li+ is −3.04 V, making lithium metal one of the strongest reducing agent available. Consequently, Li metal can reduce most known cationic species to a lower oxidation state. Because of this strong reducing capability when the lithium metal of an anode contacts a solid-state Li+ conductor containing cation components different from lithium ion, the lithium reduces the cation specie to a lower oxidation state and deteriorates the solid-state conductor.
For example, the conductor of formula:Li3PS4 contains P5+ in the formula and is thus a secondary cation to the Li+. When in contact with Li metal, reduction according to the following equation occurs (J. Mater. Chem. A, 2016, 4, 3253-3266).Li3PS4+5Li→P+4Li2SP+3Li→Li3P
Similarly, Li10GeP2S12 has also been reported to undergo degradation when in contact with lithium metal according to the following equations (J. Mater. Chem. A, 2016, 4, 3253-3266):Li10GeP2S12+10Li→2P+8Li2S+Li4GeS4 P+3Li→Li3P4Li4GeS4+31Li→16Li2S+Li15Ge4 Li10GeP2S12 contains Ge4+ and P5+ and each is reduced as indicated.
In another example, Li7La3Zr2O12, which contains secondary cations La3+ and Zr4+ undergoes chemical degradation when in contact with lithium metal according to the following chemistry (J. Mater. Chem. A, 2016, 4, 3253-3266):6Li7La3Zr2O12+40Li→Zr3O+41Li2O+9La2O3 Zr3O+2Li→Li2O+3ZrLa2O3+6Li→2La+3Li2O
Thus, many current conventionally known solid Li-ion conductors suffer a stability issue when in contact with a Li metal anode.
The inventors of this application have been studying lithium composite compounds which may serve for future use of solid-state Li+ conductors and previous results of this study are disclosed in U.S. application Ser. No. 15/626,696, filed Jun. 19, 2017, and U.S. Ser. No. 15/805,672, filed Nov. 7, 2017. However, composites of highest efficiency, highest stability, low cost and ease of handling and manufacture continue to be sought.
Accordingly, an object of this application is to identify a range of further materials having high Li ion conductivity while being poor electron conductors which are suitable as a solid state electrolyte for a lithium ion battery.
A further object of this application is to provide a solid state lithium ion battery containing a solid state Li ion electrolyte membrane.